1. Field of the Invention
The present invention relates to a near-field data communication device which is ultra small and is capable of operation with low power, and in particular to an intra-body information transfer device which transfers a signal via a human body. This application is based on a Japanese patent application (Japanese Patent Application No. Hei 8-331825 (1996)), whose description is incorporated herein as a part of this specification.
2. Description of the Related Art
With conventional near-field data communication devices using light or radio wave (airwave), there are problems in that the power consumption is large and the device itself is large. On the other hand, if the communication method uses an electrostatic field induced in a human body, it is easy to reduce the power consumption. As a communication method using an electrostatic field induced in a human body, there can be mentioned a transfer method introduced by T. Zimmerman, "Personal Area Networks (PAN): Near-Field Intra-Body Communication", Masters Thesis MIT Media Laboratory, Sept. 1995 (hereinafter referred to as the "ZIM method"). The ZIM method performs communication by transfer of high frequency-modulated signals via a route including a human body and an earth ground, and communication with reduced power consumption is made possible.
With the ZIM method however, since it includes an earth ground in a part of the transfer route of the signal, when the size of the transmitter and the receiver is made small in order to improve the portability (and wearability) of the transmitter, the coupling between the transmitter and the earth ground becomes weak, and the communicable distance becomes particularly short. This also means that down-sizing of the device is difficult with the ZIM method. In addition, there is another problem in that with a structure including the earth ground in a part of the transfer route of the signal, communication becomes impossible if the human body and the earth ground are brought into contact.
As a similar communication method, there can be mentioned a method introduced by Shigeru Tajima, "Signal Transmission Method", Japanese Patent Application, First Publication, No. Hei 7-170215 (1995) (hereinafter referred to as the "TAJ method"). With this TAJ method, only an electrode of a portion contacting with a human body is disclosed as the electrode of the transmitter and the receiver, and a structure using an earth ground is not explicitly described.
In order to perform communication using an electrostatic field induced in a human body, it is necessary to form two signal routes between a transmitter and a receiver. With the abovementioned ZIM method, the human body and the earth ground correspond to the signal routes respectively, and the existence of electrodes to connect the respective signal routes and the transmitter and the receiver is explicitly described. With the structure of a device according to the TAJ method however, there is a description showing the electrode for connection of the signal route between the transmitter-receiver and the human body, but the electrode for connection of the other signal route is not explicitly described. Hence, the two signal routes are not established, resulting in a weak coupling between the transmitter and the receiver. Thus stable communication is not performed. Namely, the TAJ method shows the possibility of communication, but it is essential to use an earth ground in order to perform good communication in a practical use. Moreover it is also described in an example in the abovementioned Japanese Patent Application publication, that good communication can be performed only when an earth ground is positively used. Furthermore, with the TAJ method, since the conductive portion of the electrode on the human body side is brought into direct contact with the human body, a metal material is in direct contact with the human body. There is thus a problem in that the human body may be badly affected (due to metal allergy and the like).
In view of the above situation, it is an object of the present invention to provide a near-field data communication device which is ultra small and capable of operation with low power.
Furthermore, it is another object of the present invention to provide a technique capable of performing communication even with a small transmitter.
Moreover, it is an object of the present invention to provide a technique capable of performing stable communication without depending upon the relationship between the human body and the earth ground, for example, a technique capable of performing communication even in a state where the human body is in contact with the earth ground.
In addition, an other object of the present invention is to provide a technique which can reduce the size of modulator and demodulator devices and suppress the consumed power.
Furthermore, an other object of the present invention is to provide a technique which can increase the output voltage with only a minimal increase in power consumption, by selecting a suitable choke coil and an LC resonator.
An other object of the present invention is to provide a technique which can perform multiple communications using the same carrier frequency by means of superposed identification numbers.
Furthermore, an other object of the present invention is to provide a technique which can transmit and receive a signal from a shock sensor mounted on the human body even in a state where a human body is in contact with the earth ground, without weakening the coupling between the transmitter and the receiver even if the transmitter of a full-time wearable input device is small.